The use of digital techniques for defining optical images is increasing rapidly in many fields including facsimile transmission, computer graphics, image enhancement, etc.
Optical images to be digitized are divided into a number of picture elements, called pixels, and a digital value representing its tonal value is assigned to each. These tonal values are then assigned positions on a gray scale into which the full range of optical densities comprising the image may be divided. Black and white images have a gray scale of two steps, and black is usually assigned a binary one, while white is a binary zero. Eight shades of gray require three binary bits as shown in table 1.
TABLE 1 ______________________________________ Gray Level Binary Value ______________________________________ 1 000 2 001 3 010 4 011 5 100 6 101 7 110 8 111 ______________________________________
The pixels in an image may be organized into a series of horizontal rows called rasters, each raster being one pixel wide. The rasters are then scanned successively by light sensing elements which, in cooperation with associated electronics, digitize the optical density of each pixel in turn to produce a series of binary digits which define the image.
Auxiliary electronics generate control codes which show where each raster begins, where the image ends, etc.
The complete bit stream, sometimes called a "bit map" may be stored in a computer memory, processed by a computer, or transmitted to a printer which can print each pixel in its proper gray scale value to reproduce the original image or an interpretation of it. This invention describes a practical and relatively low cost means of producing accurate bit maps and printing means for reproducing them.
A basic requirement for a digitizer is that a light sensing element be positioned to sense the tonal value of each pixel.
This may be done with any of a variety of "flying spot" scanners where a spot of light the size of a single pixel scans successive rasters and the light reflected from each pixel is sensed.
It may also be done by focusing successive rasters of pixels onto a linear array of sensors with a separate sensor for each pixel. The pixels can then be scanned electronically at very high rates. This technique is used in some facsimile transceivers, but the cost of the arrays is very high since each of as many as 1,728 elements must be perfect and the yield is low.
The present invention produces low cost facsimile transceivers which can be used in the home for transmission and reproduction of written messages by telephone. Here a large part of the cost is in the scanning head at the transmitter, and the printing head at the receiver.
The present invention reduces scanning head cost by using a reciprocating array of sensors wherein each sensor scans a column of pixels. In one example for a facsimile scanner, the 1,728 elements required for a fixed array are replaced by 64 elements in a reciprocating array. Here each element scans a column 27 pixels wide and the 64 elements together scan the full 1,728 pixels.
In a scanner for digitizing drawings 72" wide, 14,400 sensors can be replaced for a linear array of 144 sensors scanning 100 pixels each. Cost savings are substantial.
My copending U.S. application Ser. No. 024,608 describes a printing machine having a shuttle mechanism for moving a printing head which is similar to the mechanism for moving the scanning head of the present invention.
In one specific embodiment of this invention optical fibers of the scanner are used with appropriate light sources and controls to print on photosensitive media, and the same fibers are used at different times for scanning or printing.
In another embodiment a similar array of fibers are added, in parallel to those of the scanner, to print on photosensitive or photoconductive media.
In still further embodiments arrays of any type of non-impact printing styli are added in parallel with the scanning fibers to print on any appropriate non-impact medium. Wire styli may be used to print on dielectric media, on electrosensitive media, on thermal or electrolytic media. Magnetic heads may be used to print on magnetic media, etc.
With this printing means combined in a single moving head with the scanning means described earlier in this application, the present invention provides an effective and inexpensive facsimile transceiver which can be used at one time for digitizing an image to be transmitted and at another time for printing an image which has been received.